1. Field of the Invention
This invention relates to a catalytic converter for purifying exhaust gas from an internal combustion engine, and more particularly to a device for heating such a catalytic converter.
The present application is based on Japanese Patent Application No. Hei. 10-61400, which is incorporated herein by reference.
2. Description of the Related Art
A catalytic converter, used for purifying exhaust gas from an internal combustion engine mounted on an automobile or the like, performs an efficient exhaust gas purification effect when it is placed in a high-temperature, activated condition of not less than about 350.degree. C. Namely, in a cold condition of the engine as when starting the engine in a low-temperature condition, the exhaust gas purification properties of the catalytic converter are extremely aggravated. Therefore, the temperature of the catalytic converter need to be rapidly raised to a predetermined temperature so that the converter can be brought into a high-temperature, activated condition. Therefore, there is known, for example, a catalytic converter-heating device of the electrically-heated type in which an electric heater of the electrically-heated type is provided upstream of a catalyst unit so as to heat exhaust gas. This electric heater comprises, for example, a flat, disk-shaped honeycomb body made of metal. The electric heater is retained by an annular housing made of metal, and further there is provided a power relay means for relaying electric power to the metal honeycomb body. Preferably, a catalytic material is coated on the surface of the metal honeycomb body. Further, slits are formed in the metal honeycomb body in a zigzag manner in parallel relation to each other. When an electric current is flowed from one end at the circumferential portion of the metal honeycomb body to the other end disposed in diametrically-opposite relation thereto, a path of the current is zigzag so that the metal honeycomb body can be heated uniformly. The electric heater itself burns the unburned components in the exhaust gas under the influence of the catalytic material provided on the surface of the metal honeycomb body. And the electric heater is further heated by oxidation heat due to this burning, thereby effecting the rapid heating of the exhaust gas. Therefore, this heating device is called an electrically-heated catalyst (EHC).
FIG. 1 shows one example of a catalytic converter of the electrically-heated type. As shown in this Figure, an auxiliary catalyst unit 103 is provided upstream of main catalyst units 102 mounted in an exhaust pipe 101, and an EHC 104 is provided upstream of the auxiliary catalyst unit 103. In the catalytic converter of this construction, when an internal combustion engine is in a cold condition as at the time immediately after starting the engine, a switch SW is operated to apply, for example, a battery voltage V.sub.B to the EHC to heat the same. As a result, an electric heater (not shown) of the EHC 104 is heated to promote the oxidation of unburned components of hydrocarbons in the exhaust gas, and therefore the electric heater is rapidly heated partly with the aid of this oxidation heat, and heats the exhaust gas. As a result, the auxiliary catalyst unit 103 of the next stage is heated, and further the main catalyst units 102 are heated, so that the whole of the catalytic converter is rapidly activated.
For example, Japanese Patent Publication No. Hei. 8-218857 discloses a construction of an electric heater of the electrically-heated type for such a catalytic converter and an electrode for supplying electric power to this heater. An end portion of a lead wire for supplying electric power to the electric heater from the exterior is detachably incorporated in the electric heater, and this construction has features that handiness such as the assembling efficiency is excellent and that electrically-insulating properties and exhaust gas-sealing properties are excellent.
FIG. 2 shows a construction of an electric power supply portion of an EHC. As described above, the EHC comprises a flat, disk-shaped honeycomb structural body 2 retained in a metal housing 1 by a retaining structure (not shown). This honeycomb structural body 2 serves as an electric heater. On the other hand, an electrode receiving member 3 of a tubular shape extends through a through hole formed in the metal housing 1. An electrode rod 4 is retained in the electrode receiving member 3 by an insulating retaining member 5 in spaced relation to the inner surface of the electrode receiving member 3. The electrode rod 4 projects from the electrode receiving member 3 toward the honeycomb structural body 2. A distal end of the projected portion of the electrode rod 4 is electrically connected to a flexible connecting member 6 made of metal while the other end of the electrode rod 4 is electrically connected to a sheathed lead wire 8 having an insulating sheath 7. A waterproof member 10 is interposed between the sheathed lead wire 8 and an electrode-retaining tubular member 9, and is compressively clamped from the outside. The retaining member 5 comprises a first insulating member 5a and a second insulating member 5b both of which are made of a ceramics material of an inorganic material, and insulating powder 5c of an inorganic material filled in a space formed between the first and second insulating members 5a and 5b. With this construction of the retaining member 5, the vibration of the electrode rod 4 due, for example, to impingement of solid materials flying from the outside of the exhaust pipe, as well as a heat cycle developing in the electrode rod 4, can be absorbed by the powder 5c, and therefore an airtight seal between the outside and inside of the exhaust pipe 101 can be maintained.
The power relay means for feeding electric power to the honeycomb structural member 2 is formed by the above construction.
Generally, exhaust gas from an internal combustion engine contains pyrolytic high molecular substances and free carbons due to engine oil, and abrasion powder resulting from a cylinder wall and a piston ring. These foreign matters adhere to the projected portion of the electrode and the honeycomb structural member which are exposed directly to a stream of exhaust gas. Particularly when the electrically-conductive foreign matters adhere to and deposit on the projected portion of the electrode and its vicinity, this deteriorates the ability of insulation between the housing portion (the grounded side) and the electrode rod having a potential difference relative thereto.